Composite materials are increasingly used in modern aircraft to inc-ease the strength to weight ratio, load capacity and performance characteristics thereof. In some modern aircraft, composite materials such as graphite fiber/epoxy resin composites have replaced metal components as structural elements of the aircraft. As composite materials have replaced metal materials, new fabrication techniques for making composite material structures have been developed.
There are two generic classes of plastic resins which are used in combination with reinforcing fibers of the composite material. One class of plastic resins is known as thermosetting resins, the other class is known as thermoplastic resins. Thermoplastic resin/fiber reinforced composite materials are characterized in that the resins are reformable under heat after the resins have been cured. Composites made from thermosetting resins and reinforcing fibers cannot be reformed once they have been cured. There are also differences in the manner of handling and forming laminates from these different classes of materials.
Thermosetting techniques are most familiar to the general public and are often employed in the construction of fiberglass boats. Composites which use a thermosetting resin are typically supplied on large rolls in the form of tape. The uncurved resin is tacky at room temperature. Therefore, the tape is supplied with a backing which permits the material to be coiled on a roll. This tackiness or "drape" of the thermosetting composite materials is advantageous in many applications. For example, thermosetting composite materials can be placed upside down in a mold and will remain in place.
Thermosetting materials are typically cured in an autoclave under elevated temperatures and pressures. In a conventional autoclave technique, layers of thermosetting resin, pre-impregnated, fibrous material, are layed-up on or around a forming tool. Air pockets can be squeegeed or rolled out of the laminate layers as the layers are applied. Air does not reenter the laminate between the layers because the layers are sticky. After the uncured laminate has been layed-up, breather layers are positioned around and above the laminate. A microporous film, commonly referred to as a vacuum bag, is then positioned over the uncured laminate, breather layers and is sealed to the forming tool. The vacuum bag is evacuated and the entire assembly is cured at elevated temperatures and pressures in an autoclave. The use of thermosetting resins is particularly well adapted to the autoclave processing technique because the uncured laminate is substantially void free and has approximately the same thickness as the resulting cured laminate. This process is also applicable to very large parts as relatively large autoclaves are presently available.
In contrast to the above, composite materials made with thermoplastic resins are somewhat difficult to handle. The thermoplastic resins are at least partially cured when supplied in the rolled form. The laminate are not tacky at room temperature as are laminate made with thermosetting resins. The laminae tend to slide relative to one another when positioned in a laminate stack. Therefore, air pockets can not be squeegeed or rolled out of the laminate layers as the layers are applied. Furthermore, the layers often retain a slight curvature from the roll and tend to stack up in a bulky stack with many air gaps. For example, in a 24-ply laminate of thermoplastic resin pre-impregnated composite material, the unconsolidated laminate will have a thickness of approximately four inches. The ultimate, desired thickness for the consolidated thermoplastic laminate is only approximately 0.12 inch. The conventional autoclave method described above does not satisfactorily debulk the laminate stack.
Thermoplastic laminates are typically formed by the press-work method because of the above described limitations of the conventional autoclave technique. The press-work method involves the use of a two part mold which, when assembled, forms a void having the ultimate, desired shape of the composite part. Layers of composite material are placed in the mold. The mold halves are then forced together and heated to a high temperature to compress and consolidate the layers. This method has proven satisfactory for producing small parts in high volume. However, economic disadvantages are associated with the press-work method, if relatively few parts are made, or if the parts are extremely large requiring very large molds and presses.
As stated above, composite materials have replaced metal, even in structural applications for aircraft, such as frame members and stringers. Stringers may be in the form of I-beams or T-beams of up to 40 feet long or longer. A great advantage can be obtained if thermoplastic materials are used for construction of these beams because the thermoplastic structural element can be reformed by local application of heat to correct manufacturing errors or deviations in manufacturing tolerances.
Thus, a need exists for a method for preparing large, composite structures from thermoplastic materials which does not require the use of expensive molds or large presses.